Femtosecond IR pulses

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

V. S. Letokhov My answer to Prof. Quack is that it is indeed difficult to predict theoretically the effect of intense femtosecond IR pulses on the IVR rate of polyatomic molecules, which is important for the transfer of vibrationally excited molecules from low-lying states to the vibrational quasi-continuum. We are developing the relevant theoretical mechanisms of IR MP E/D of polyatomics since the discovery of this effect for isotopic molecules BC13 and SF6 in 1974-1975.1 hope that it will become more realistic to study experimentally the influence of intense IR pulses on IVR due to the great progress of femtosecond laser technology. 

We are beginning to develop a detailed understanding of these methods (18,21,30,33,34,37-40,42,44,47-49), many of which are described in this book. We have recently demonstrated a series of novel nonlinear all-IR spectroscopic techniques (IR-pump-IR-probe, IR-three-pulse photon echoes, IR-dynamic hole burning, IR-2D spectroscopy), all of them utilizing intense femtosecond IR pulses, with the intention to develop new multidimensional spectroscopic tools to study the structure and the dynamics of proteins (30,31,41,42,50-53). We shall summarize in this contribution our work, its underlying principles, and its applications. 

Following a two photon excitation of hydrated hydroxyl ions (FhO/NaOH = 55) with femtosecond UV pulses (/.purnp = 310 nm, Eexdtation = 2x4 eV), short-time electron transfer trajectories have been investigated by near-IR and UV absorption spectroscopies at room temperature. The energy of the pump beam is 1011 W cm 2. 

Details of the laser systems for pump-probe experiments are described elsewhere [8,10], except for a femtosecond IR probe system. For probing IR wavelengths (5 10 pm), a regenerative amplifier system of a Ti sapphire laser (800 nm wavelength, 160 fs FWHM pulse... 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

S. Tzortzakis, B. Prade, M. Franco, A. Mysyrowicz, Time evolution of the plasma channel at the trail of a self-guided IR femtosecond laser pulse in air, Optics Commun. 181, 123 (2000)... 

Finally, time-resolved spectroscopy with femtosecond pulses was recently carried out by Gale and coworkers on a similar HD0 D20 sample (125). Due to the notably wider bandwidth of the applied IR pulses in the latter investigations, no details on reshaping of the transient spectra in dependence of the excitation frequency were accessible. A time-dependent position of the peak position of the induced sample bleaching was interpreted in terms of a shift within the statistical distribution of OH frequencies with a time constant of 1 ps. However, because only the parallel signal of the induced sample transmission was detected, the measured dynamics corresponds to a superposition of vibrational, reorientational, and structural relaxation. The data are interpreted by the help of a model of with random (bell-shaped) distribution of OH oscillators, quite different from the results of other groups. 

To experimentally probe the CO trajectory after dissociation, ultrafast time-resolved polarized mid-IR spectra of photolyzed h-MbCO in G/W were recorded (34), the results of which are plotted in Fig. 8A. This study was performed in G/W primarily because the flatness of the solvent absorbance spectrum near 2100 cm-1 minimizes temporal distortion of the transmitted femtosecond IR probe pulse, thereby maximizing the effective time resolution of the measurement. Two features are already apparent at 0.2 ps, the earliest time shown, and these features rapidly develop into the docked states denoted Bi and B2. The development of the docked CO spectrum is further quantified by the time dependence of the polarization anisotropy, as defined in Equation (2). The B and B2 polarization anisotropies, plotted in Fig. 8B, evolve exponentially with time constants of 0.20 0.05 ps and 0.52 0.10 ps, respectively, and converge to the same anisotropy of approximately 0.2. According to Fig. 8C, ligand translocation is accompanied by a 1.6 0.3 ps growth of the integrated isotropic B-state absorbance. 

The generation of intense femtosecond mid-IR pulses was performed in two frequency conversion steps (see Fig. 1), similar to the setup described in Ref. 57 ... 

SFG using femtosecond lasers allows all the resonances within the broad (-200 cm" ) bandwidth of the IR pulse to be probed simultaneously, without scanning the infrared source. To obtain spectral resolution in an SFG spectrum, the IR polarization is upconverted with a narrowband (-8 cm" ) visible beam, which is prepared by pulse shaping the output of a femtosecond laser. Only the frequency components of the pulse that interact resonantly with the vibrational modes are enhanced, resulting in an SFG spectrum [28, 29]. Owing to the use of femtosecond... 

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